Deep hardening steel

ABSTRACT

A DEEP HARDENABLE, LOW ALOY STEEL HAVING GOOD TEMPER RESISTANCE, HOT HARDNESS AND DIMENSIONAL STABILITY CONSISTING ESSENTIALLY OF ABOUT 0.7-1.2% CARBON, UP TO 0.6% MANGANESE, 0.5-2.5% SILICON, 0.5-1.5% CHROMIUM, 0.25-1.5% MOLYBDENUM, 0.15-0.5&amp; NICKEL, 0.65-4% COPPER AND THE BALANCE ESSENTIALLY IRON EXCEPT FOR INCIDENTAL IMPURITIES, AND IN WHICH 8.5X%SI+3X%CU-5.2$10.

United States Patent 3,712,808 DEEP HARDENING STEEL Thoni V. Philip, Reading, Pa., assignor to Carpenter Technology Corporation, Reading, Pa. No Drawing. Filed Nov. 26, 1971, Ser. No. 202,651 Int. Cl. C22c 39/54 US. Cl. 75-125 5 Claims ABSTRACT OF THE DISCLOSURE A deep hardenable, low alloy steel having good temper resistance, hot hardness and dimensional stability consisting essentially of about 07-12% carbon, up to 0.6% manganese, 05-25% silicon, 0.51.5% chromium, 0.25l.5% molybdenum, 0.15-0.5% nickel, 0.65-4% copper and the balance essentially iron except for incidental impurities, and in which 8.5X %Si+3 %Cu5.210.

This invention relates to alloy steel and, more particularly, to such steel having a relatively small alloy content and characterized by deep hardenability, good temper resistance, hot hardness and dimensional stability.

AISI-SAE type 52100 steel having a nominal composition of about 1.0% carbon, 0.30% manganese, 0.25% silicon, 1.4% chromium, and the balance iron except for incidental impurities has long been used in the manufacture of bearings but is characterized by relatively low hardenability and poor resistance to tempering. Bearing parts of AISIASAE 52100 steel are frequently through hardened by oil quenching from a hardening temperature of about 1550 F. followed by tempering at about 350 F. to provide a heat-treated hardness of about 61 to 63 on the Rockwell C (R scale. In order to maximize dimensional stability of such parts, tempering is carried out at a temperature of about 450 F., but this results in a heat-treated hardness of only about R 59-6l. Because of its low temper resistance, that is to say the loss in hardness that rapidly occurs when such parts are exposed to temperatures above about 450 F., bearing parts made of 52100 steel should not be operated at temperatures above about 400 F. Another drawback of the 52100 steel resides in the fact that because of the limited depth to which it can be hardened, bearing parts only about inch or less in thickness can be through hardened by quenching in oil. While parts having a somewhat greater thickness can be through hardened by quenching in water, there is considerable risk that the parts will become warped or even cracked.

When greater hardenability is desired for the manufacture of parts such as gears having a thickness greater than about inch, modifications of 52100 steel containing larger amounts of manganese and silicon have been hitherto used. One such steel has the designation SKF #1 and a nominal composition of about 0.95% carbon, 1.1% manganese, 0.6% silicon, 1% chromium and the balance iron except for incidental impurities. Another has the desig nation SKF #2 and a nominal composition of 0.9% carbon, 1.5% manganese, 0.7% silicon, 1.5% chromium, and the balance iron except for incidental impurities. Although both SKF #1 and SKF #2 have better hardenability than the 52100 steel with the SKF #2 steel better than the SKF #1, they do not provide any significant improvement in temper resistance as compared to the 52100 steel.

Another bearing steel, hitherto known by the designation SZCB has a nominal analysis of about 0.85% carbon, 0.35% manganese, 0.8% silicon, 1% chromium, 0.55% molybdenum and the balance iron except for incidental impurities. It has considerably improved temper resistance and can be operated at temperature as high as about 550 F. But the 52GB steel is limited by its hardenability for use as gears no thicker than about 1 inch when quenched in oil from a hardening temperature of about 1575 F. While slightly higher hardening temperatures can be used to provide greater hardenability, the likelihood of warping and cracking is increased as Well as the difiiculty of shaping the parts as by grinding.

M50 type high speed tool steel has been successfully used to provide bearings for use at temperatures of up to about 700 F. This steel has a nominal composition of about 0.85% carbon, 4% chromium, 4% molybdenum, 1% vanadium and the balance iron except for incidental impurities. Besides requiring hardening from the undesirably high temperature of 2000 F., M50 steel contains a relatively high level of costly alloying additions which render it relatively expensive.

It is therefore a principal object of this invention to provide a low alloy steel having improved deep hardenability and temper resistance.

Another object of this invention is to provide such an alloy steel which has good hot hardness and dimensional stability.

A more specific object is to provide such an alloy steel which can readily be quenched from a relatively low hardening temperature and then tempered to provide dimensionally stable, deep hardened parts having good temper resistance and hot hardness.

The foregoing objects as Well as additional advantages of this invention are achieved by providing an alloy steel which in its broad and preferred forms consists es- The remainder of the alloy is iron except for incidental impurities which may vary from a few hundredths of a percent or less as in the case of sulfur and phosphorus or up to one-quarter or one-half percent as in the case of those elements such as aluminum, columbium, titanium, vanadium, Zirconium and calcium which are used as deoxidizers and/or grain refiners. The larger amounts of the carbide or carbonitride formers such as titanium, columbium or zirconium may be better tolerated at the higher levels of carbon. It is to be noted that it is not intended to restrict the ranges indicated by presenting them in tabular form for ready reference. It is contemplated that when desired, any one or more of the preferred ranges indicated can be used with any one or more of the broad ranges indicated for the remaining elements.

In the composition of the alloy steel of this invention, a minimum of about 0.7 carbon is required to provide the high heat-treated hardness required in bearing parts and to ensure the presence of dispersed residual, preferably fully spheroidized, carbides to impart the wear resistance that may be needed. Because carbon is a powerful austenite former, higher carbon levels than about 1.2% may result in excessive amounts of retained austenite in the heat-treated bearing parts formed from the alloy which would make it difficult, if not impossible, to attain adequate hardness for an intended use. It is essential for the attainment of dimensionally stable bearing parts that retained austensite not exceed about 4%. Preferably about 0.85-0.95% carbon is present in the steel.

Manganese is a commonly used alloying addition for increasing hardenability, but because of the difliculties caused when vacuum techniques are employed in melting it is preferably limited to residual levels, that is no more than about 0.4%. Up to about 0.6% manganese can be present in the alloy but at higher levels, manganese leads to excessive amounts of retained austenite in the as-heattreated microstructure which in turn causes objectionable dimensional instability for such products as hearing parts.

In this alloy, chromium contributes to hardenability, provides resistance to oxidizing media, and minimizes scale formation when the alloy is hot worked. To this end a minimum of 0.5% chromium is required, while above about 1.5% chromium, the alloy requires hardening at temperatures so high as to make it difiicult to quench and also may result in the retention of excessive amounts of austenite. Preferably about 0.75% to 1.25% chromium is present in the alloy.

When present in amounts greater than about 1.5%, molybdenum, like chromium, adversely affects the hardening temperature of the alloy and also may cause the retention of excessive amounts of austenite. Thus, about 0.25% to 1.5%, preferably 0.5% to 1%, molybdenum is used for its beneficial effect upon the hardenability of the alloy.

Silicon and copper work together to provide the outstanding combination of hardenability, toughness and temper resistance characteristics of this composition. To this end at least 0.5 silicon, better yet 0.7% and up to 2.5%, and 0.65-4.0% copper are present in the alloy. Silicon functions as a ferrite strengthener. When silicon is present above about 2.5 graphitic carbon is formed in the alloy and and adversely affects the hardness as well as objectionably raising the hardening temperature of the alloy. Copper, on the other hand, is an austenite former. When copper is present in amounts above about 4%, precipitation of some of the copper may occur if the alloy is maintained at temperatures of about 7004000 F. or above for an appreciable time. Preferably silicon ranges about 1.5-2.0% and copper about 1.5-3.0%.

A minimum combined content of silicon and copper is required if the properties of this alloy are to be consistently attained which are required to produce bearing parts characterized by deep hardenability and having good temper resistance at temperatures as high as 700 F. The highly advantageous properties of this alloy are attainable when silicon and copper are balanced so that 8.5 times the percent silicon plus 3 times the percent copper less 5.2 are at least equal to 10. That is to say When silicon and copper are thus balanced, the equation gives a close approximation of the Jominy distance at which the as-heat-treated hardness is no less than R 63.

A small but essential amount of nickel is present in this alloy because of its effect on the hot workability of the alloy. A minimum of about 0.15% is required to ensure the avoidance of tears which would otherwise occur during hot working. Up to about 0.5 nickel can be used for this purpose, but care must be used because if too much is used difficulties in ennealing and machining as well as excessive amounts of retained austenite may be encountered. Preferably 0.20-0.35 nickel is used.

This alloy is readily prepared by means of conventional, well-known techniques. Because elements such as manganese which are difiicult to control when an alloy is melted under low pressure, are not relied upon to provide the desired properties, this alloy can readily be melted and then remelted from a consumable electrode under low pressure. Parts can be forged from a furnace temperature not over about 2050 F. and should be slow cooled in a furnace or a suitable material such as in dry ash or vermiculite, to room temperature. Annealing can be carried out by heating to about 1425 to 1475 F. for up to about hours, depending upon the size of the part, cooling slowly in the furnace at the rate of about 10 to F. per hour to at least about 1100 F. and then air cooling to room temperature. Hardening can be carried out by quenching in oil from the low hardening temperature of 1500 F. although temperatures up to about 1550" F. can be used. Within about 4 hours after the part has been quenched to room temperature, it is cooled to at least about 105 F. and then slowly permitted to return to room temperature.

Depending upon the intended use, parts are tempered by heating for up to 4 hours at the required temperature of from about 350 F. to about 700 F., tempering being carried out at least at or about 50 to 75 above the highest sustained temperature expected in service.

Example 1 As an example of the present invention, a seventeen pound vacuum induction heat having the following analysis in weight percent was melted and cast as a 3% inch square ingot:

and the balance was iron except for incidental impurities which included less than about 0.005% each of phosphorus and sulfur. The ingot was forged by pressing to a 1 /8 inch square billet from a furnace temperature of 2050 F. and buried to cool. The billet was annealed by heating for 4 hours at 1450 F. and then cooled 15 F. per hour down to at least about 1100 F. and then allowed to cool in air. In that condition, specimens gave a hardness of R 101. Thus annealed specimens when austenitized at 1500 F. for 15 minutes, oil quenched and then refrigerated for 15 minutes in liquid nitrogen had a hardness of R,, 66 and no more than about 4% retained austenite. A thus heat-treated specimen, after being tempered at 500 F. for 4 hours followed by cooling in air, exhibited a hardness of R 62.5 and about 4% retained austensite. Tempering at 600 F. for 4 hours followed by cooling in air gave a hardness of R 61.5 and 3% retained austenite.

Specimens of the alloy of Example 1, after heating at 1500 F. for 15 minutes, were quenched in oil, refrigerated in liquid nitrogen, tempered at 600 F. for 4 hours and then air cooled. Following such heat treatment, the hot hardness of the as-heat-treated and tempered hardness was measured at room temperature (R.T.), 200 F., 300 F., 400 F., 500 F. and 600 F. with the results indicated in Table I.

For purposes of comparison, 17-pound vacuum induction heats of each of the hitherto used alloys SKF #1,

SKF #2 and 5203 were prepared having the following analysis in weight percent:

TABLE II 0 Mn Si Cr Mo The balance was iron in each case except for incidental impurities which included less than 0.005 each of phosphorus and sulfur. Test speciments were prepared therefrom as described in connection with the alloy of Example 1 except that the heat treatment best suited to these alloys were used instead of that used for Example 1.

SKF #1 specimens austenitized at 1525 F. for 15 minutes and oil quenched had a hardness of R 65 and 14% retained austenite, and then when tempered at 350 F. for 1 hour followed by cooling in air had a hardness of R 61.5 and 14% retained austenite.

SKF #2 specimens austenitized at 1500 F. for 15 minutes and oil quenched had a hardness of R 63.5 and 8% retained austenite, and then when tempered at 350 F. for 1 hour followed by cooling in air had a hardness of R 60 and 10% retained austenite.

Specimens of 52GB austenitized at 1525 F. for minutes, oil quenched and refrigerated for 15 minutes in liquid nitrogen had a hardness of R,, 64.5 and 4% retained austenite, and then when tempered at 600 F. for 4 hours and air cooled had a hardness of R 58.5 and 1% retained austenite.

As in the case of Example 1, the hot hardness of specimens of the alloys SKF #i1 and #2, and 52GB were measured on tempered specimens, and the results are indicated in Table III where the hot hardnesses obtained from the alloy of Example 1 have been repeated to facilitate comparison. In the case of SKF #1 and #2, hot hardnesses of necessity were only measured up to 300 F. because of the lower tempering temperatures that are usable with those two allo'ys.

TABLE III 1 1 Hardness-Rs scale.

The depth of hardenability of the alloy of Example 1 was found to be greatly superior to SKF #1, SKF #2 and 52GB as measured by the Jominy end-cooled test of A.S.T.M. Specification No. A-255. Briefly stated, the test involves heating a test bar 1 inch in diameter to the appropriate austenitizing temperature and then cooling only the end face with water under controlled conditions. The hardness is then measured at intervals ,4 inch from the water-cooled end to determine how far from the watercooled end the increased hardness extends. In Table IV, the hardnesses measured from test bars formed from each of the four analyses are listed for each of the li inch Jominy intervals indicated in the left-hand column.

TABLE IV Jominy dist. (Me in.) SKF #1 SKF #2 52GB Example 1 1 HardnessR scale.

I The hardness actually measured was Ro 61.9, an obvious error presumably caused by the closeness to the end face. Therefore, the value of R 65 is indicated and is believed correct to within less than =1=.5, the usual experimental error in such measurements.

Example 1 is clearly seen to be superior to the other alloys in depth of hardenability. At Jominy 9, the SKF #1 specimen tested has fallen below the R 58 hardness usually required for hearing parts. This occurs at about Jominy 15 or 16 for the SKF #2 and 52GB specimens. On the other hand, the specimen of Example 1 gave a hardness of R 61.4 at Jominy 16 or one inch from the cooled face.

The coefiicient of thermal expansion was measured on the alloy of Example 1 over the temperature ranges indicated in Table V extending from 75-200 F. to 75 The alloy of the present invention is especially well suited for use as a bearing steel in the manufacture of bearing parts requiring deep hardenability, high as-heattreated hardness, good resistance to tempering, hot hardness and dimensional stability. It is also suited for use in making parts having an intricate shape. In addition to its improved properties, the alloy of the present invention has the further important advantage that it can be produced at considerably less cost than an alloy such as type M50.

As has been pointed out hereinabove, for good dimensional stability the heat-treated and tempered parts made from the alloy should have an essentially martinsitic microstructure with" no more than about 4% retained austenite. On the other hand, when the contemplated use of the parts formed from the alloy does not require dimensional stability, as when the part will be subjected to use only at room temperature, then as much as 20% retained austenite could be tolerable. In that event refrigeration could be omitted from the heat treatment cycle, and tempering could be carried out at a much lower temperature, eg at about 350 F. for 1 hour followed by cooling in air.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. A deep hardenable low alloysteel which as hardened at about 1500 F., oil quenched, refrigerated and tempered at at least about 500 F., has good temper resistance, hot hardness, dimensional stability and contains no more than about 4% retained austenite, which is suitable for making bearing parts for use at elevated temperatures and which by weight consists essentially of about Percent Carbon 0.7-1.2 Manganese Up to 0.6 Silicon 0.5-2.5 Chromium 0.5-1.5 Molybdenum 0.251.5 Nickel 0.l5-0.5 Copper 0.65-4.0

2. The alloy steel as set forth in claim 1 which contains no more than about 0.4% manganese.

3. The alloy steel as set forth in claim 1 which lontains at least about 0.7% silicon.

4. The alloy steel as set forth in claim 1 which contains about 1.52.0% silicon, and about 1.53.0% copper.

5. The alloy steel as set forth in claim 1 which contains about OBS-0.95% carbon, up to about 0.4% manganese, about 1.52.0% silicon, about (HS-1.25% chromium, about 0.501.0% molybdenum, about 0.20-0.35% nickel, and about 1.53.0% copper.

References Cited UNITED STATES PATENTS 2,565,953 8/1951 Gaspari 75-125 5 3,060,016 10/1962 Melloy 75-425 3,330,652 7/1967 Robinson 75-125 3,499,757 3/1970 Mandich 75 125 HYLAND BIZOT, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 112595 I Dated January 23, 1973 Inventor(s) 'I'honi V. Philip It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 30, delete the second "and".

Column 3, line 58, "ennealing" should be annealing Column 4, line 16, "3 3/4" should be 2 3/4 E Column 6, line 59, "alloysteel' should be alloy steel Column 7, line 6, "lon" should be con- Signed and sealed this 7th day of May 197L V (SEAL) Attest:

EDWARD M.FLETCHER,JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents $69) USCOMM-DC 60376-P69 U.S. GOVERNMENT PRINTING OFFICE 2 1," 3J3 

